1. Field of the Invention
This invention relates in general to the field of pressurized water nuclear reactors and in particular to a fuel assembly for a pressurized water reactor employing a fluid moderator system for purposes of spectral shift control.
2. Description of the Prior Art
In typical pressurized water nuclear reactors, control over the fission process, or reactivity control is accomplished during reactor operation by varying the amount of neutron-absorbing materials within the core of the reactor. One method to effectuate reactivity control is by the use of control rods containing such neutron absorbing materials or poisons which are inserted within the reactor core. Control over the fission process may be accomplished by varying the number of control rods, the size of the control rods and their radial and axial locations within the core. Burnable poisons (by the fissioning process) and poisons dissolved in the reactor coolant can additionally be used for purposes of such control.
In order to lengthen the core life, it is typical, in conventionally designed commercial pressurized water reactors, to design in an excess of reactivity at reactor start-up. The excess reactivity is controlled as stated above, and is gradually depleted over the extended life of the core. Soluble boron, dissolved in the reactor coolant is most often used to control the initial excess reactivity. As the excess reactivity in the core is depleted during the reactor operation, the neutron absorbing boron is gradually removed so as to utilize the original excess reactivity to maintain the fission process. While this control arrangement provides an effective means of controlling a nuclear reactor over an extended core life, the neutron absorbing boron used during core life absorbs neutrons and removes reactivity from the reactor core that could otherwise be used in a more productive manner. For example, the reactivity could be used to convert fertile material to plutonium or to fissile uranium which even further extends the reactor core life by fissioning the then generated fissile material. Without such conversion, however, the consumption of reactivity is an inefficient depletion of uranium resulting in higher fuel costs than would otherwise result. In view of the above, it would be an obvious advantage to be able to extend the life of a core having an initial amount of excess reactivity while not suppressing the excess reactivity with neutron absorbing materials, but rather using the excess reactivity in a positive manner thereby providing an extended core life with a significantly lower overall fuel cost.
It is well known that fuel element enrichment can be reduced and the conversion ratio of producing fissile materials can be increased by employing a "hardened" (higher neutron energy) spectrum during the first part of the fuel cycle to reduce excessive reactivity and to increase the conversion of fertile material to fissile material; then employing a "softer" (lower energy) neutron spectrum during the latter part of the fuel cycle to increase reactivity and extend the core life by fissioning the previously generated fissile material. One such method utilizing the above is known as spectral shift control which provides a reactor with an extended core life while reducing the amount of neutron absorbing material in the reactor core. In this art, the reduction of the excess reactivity, and, therefore, the neutron absorbing material, is achieved by replacing a portion of the ordinary reactor water with heavy water.
The heavy water is a less effective moderator than the ordinary reactor coolant water. This retards the chain reaction by shifting the neutron spectrum to higher energies permitting the reactor to operate at full power with reduced neutron absorbing material. This shift to a hardened neutron spectrum causes more fertile U-238 or Th-232 to be converted to fissile Pu-239 or U-233, respectively, that may thereafter be consumed in the reactor core producing heat and further extending the core life. Thus, the shift to an initially hard spectrum results in more neutrons being consumed in a useful manner rather than being wasted by the use of poisons. As the fissile material is consumed, the heavy water is gradually replaced with ordinary reactor coolant water creating a softer neutron spectrum whereby the core reactivity is maintained at a proper level. At the end of core life, essentially all of the heavy water has been replaced by the ordinary reactor coolant water. Thus, the reactor can be controlled by control rods without the use of additional neutron absorbing material and without the use of excess reactivity at start-up, resulting in significant uranium fuel cost savings. The additional Pu-239 or U-233 production also reduces the U-235 enrichment requirements.
While the spectral shift control is well known in theory, there exists a need to carry the theory into effect. To date, no apparatus exists which effectively and practically implements such theory.
It is, therefore, a primary object of the present invention to provide a fuel assembly for a pressurized water nuclear reactor which includes means for varying the reactor coolant water volume over a fuel cycle by replacing a portion of the water with heavy water during the early stages of core life and then gradually reducing the amount of heavy water and replacing it with ordinary water as the core life decreases.